The existing descent control devices for fire escape use centrifugal friction mechanism to enable a person to descend at a constant speed. These descent control devices are often out of control, cause casualties and thus have low reliability. The related art also discloses a descent control device with constant speed, which solves the problems of the descent control device with centrifugal fiction mechanism. But this descent control device with constant speed uses a planetary drive wheel train whose gear ratio is low and hard to change. So the force which the slip component is subjected to is large. And the slip component is easy to breakdown first, and the service life of components of the descent control device will not be the same with each other. And it is hard to adjust the decline speed of the human body as needed.